Dark frames are taken rarely with 2x2 bin, low and high gain. Those
with low gain usually have neglible dark current in ADU. The
typical exposure time is 1800 sec. The FORS pipeline generates master
DARKs from input raw DARKs with master BIAS subtracted.
Dark current: is computed from the median master dark frame level
in ADU/hr/px and transformed into electrons by using the conversion
factor CONAD. More information can be found on
the plot tutorial page